Electrochemical Carbon Dioxide Reduction in Methanol at Cu and Cu2O-Deposited Carbon Black Electrodes

The electrochemical reduction of carbon dioxide in methanol was investigated with Cu and Cu2O-supported carbon black (Vulcan XC-72) nanoparticle electrodes. Herein, Cu or a Cu2O-deposited carbon black catalyst has been synthesized by the reduction method for a Cu ion, and the drop-casting method was applied for the fabrication of a modified carbon black electrode. A catalyst ink solution was fabricated by dispersing the catalyst particles, and the catalyst ink was added onto the carbon plate. The pH of suspension was effective for controlling the Cu species for the metallic copper and the Cu2O species deposited on the carbon black. Without the deposition of Cu, only CO and methyl formate were produced in the electrochemical CO2 reduction, and the production of hydrocarbons could be scarcely observed. In contrast, hydrocarbons were formed by using Cu or Cu2O-deposited carbon black electrodes. The maximum Faraday efficiency of hydrocarbons was 40.3% (26.9% of methane and 13.4% of ethylene) at −1.9 V on the Cu2O-deposited carbon black catalyst. On the contrary, hydrogen evolution could be depressed to 34.7% under the condition.


Introduction
The increasing carbon dioxide (CO 2 ) greenhouse gas concentration in the atmosphere has created serious problems associated with the energy supply based on fossil fuel [1,2]. Therefore, it is very significant to decrease the atmospheric CO 2 concentration, and a promising approach is a C-based fuel technique, which is obtained from abundant sources containing CO 2 by applying the renewable energy as energy input. The electrochemical CO 2 reduction appears to be one potential of the most important methods for the CO 2 conversion, because of its simplicity, environmental friendliness, and low cost [3][4][5][6][7][8][9].
The reduction products can be roughly controlled by the metallic material type of electrode. Four distinct groups for the metal catalyst have been identified for the electrochemical reduction of CO 2 [8]: (1) metals that mainly produce HCOOH (Pb, Hg, Cd, In, Sn, and Tl), (2) metals that mainly produce CO (Au, Ag, Pd, Ga, and Zn), (3) metals that form hydrocarbons such as CH 4 and C 2 H 4 (Cu), and (4) metals that mainly produce hydrogen, H 2 (Pt, Ni, Fe, and Ti).
Methanol (MeOH) is a better medium for CO 2 than water. The solubility of CO 2 in methanol is more than about four times that in water, at ambient temperature [10]. Therefore, in the Rectisol method, methanol has been used as a physical absorbent of CO 2 on an industrial scale [10]. Recently, over 70 large-scale plants have applied the Rectisol process. Therefore, the electrochemical CO 2 reduction in methanol may become an energetically efficient method.
Recently, the electrochemical reduction of CO 2 at a modified carbon material electrode has been investigated. Carbon blacks have been widely applied into the various fields of electrochemistry due to their good electrical properties, high mesoporous distribution, low cost, and high availability [11]. Various carbon blacks containing acetylene black, Black Pearl, Ketjen black, and Vulcan XG-72 have been applied into the electrodes for supercapacitors or catalyst supports in the electrochemical fields.
Baturina et al. [12] reported the activities of Cu nanoparticles supported on single-wall carbon nanotubes (SWNTs), carbon black (Vulcan XC-72R), and Ketjen Black (KB) for CO 2 electroreduction using a sealed rotating disk electrode (RDE). Moreover, the same research group [13] investigated the effect of support on the electrocatalytic activity of Cu nanoparticles for the CO 2 electroreduction using three types of nanostructured carbons: onion-like carbon (OLC), single-wall carbon nanotubes (SWNT), and reduced graphene oxide (RGO). Shinagawa et al. [14] studied the CO 2 electrochemical reduction in aqueous media with the size-controlled carbon-supported nanometric CuS catalysts. Furukawa et al. [15] explored the CO 2 electroreduction activities of Pt-Ru sputtered Pt-Ru/C powder electrodes under acidic conditions. Yin et al. [16] showed that Pd-Cu bimetallic alloy nanoparticles on a C support have excellent catalytic activity with high selectivity to CO. Zhang et al. [17] tested nanostructured tin catalysts loaded onto high surface area carbon supports for selective electrochemical CO 2 reduction. Geioushy et al. [18] investigated the efficient electrochemical CO 2 reduction on graphene-supported Cu 2 O electrodes. Cao and Wen [19] studied the Cu nanoparticles decorating reduced graphene oxide nanohybrids for the electrocatalytic CO 2 reduction. Qin et al. [20] investigated the chloride-derived copper-modified carbon paper electrode for electrochemical CO 2 reduction. Guo et al. [21] explored the efficient and tunable CO 2 reduction at Co 3 O 4 -carbon dots-C 3 N 4 catalysts. Pérez-Rodríguez et al. [22] tested the electrochemical reduction of CO 2 in water at various non-noble metal (Ni, Cu, Co, and Fe) catalysts deposited on commercially carbon black (Vulcan XC-72R).
Since water is the solvent acting as the medium in the electrochemical CO 2 reduction at modified carbon material electrodes in all of these previous reported works, there is little information on the modified carbon material electrodes in the organic solvents. Therefore, the present work has mainly dealt with the electrochemical reduction of CO 2 in methanol at Cu and Cu 2 O-deposited carbon black (Vulcan XC-72R) electrodes.

Materials and Instruments
All of the chemicals that were used were of analytical grade. Copper nitrate (>99%), sodium tetrahydroborate (>95%), and sodium hydroxide (>97%) were obtained from FUJIFILM Wako Pure Chemical Corp. and were used as received without further purification. Nafion (5 wt% in propanol) and isopropanol (>99%) were obtained from Sigma-Aldrich Co. Carbon black (Vulcan XC-72) was purchased from Cabot Coop. Carbon plate (thickness of 0.50 mm, 99.5%) was purchased from Nilaco Corp. The pure water was obtained from an ultrapure water system (Advantec MFS Inc.) resulting in a resistivity >18 MΩ cm. For the characterization of Cu species, an X-ray diffractometer (XRD, RIGAKU Ultima IV, sample horizontal type) was used with Cu K α radiation (λ = 0.15406 nm) at 40 kV and 50 mA. The deposition of Cu species into carbon black was analyzed by scanning electron microscope (S-4000, Hitachi) and transmission electron microscope (H-700H, Hitachi).

Catalyst Preparation
Cu and Cu 2 O-deposited carbon black catalysts were prepared by a reduction method with a reducing agent. For metallic copper loading, copper nitrate (0.076 g) and carbon black (0.030 g) was added to 20 mL of water; then, the suspension solution was subjected to the ultrasound. The pH of suspension solution was about two. Then, in order to reduce the copper ion and deposit metallic Cu onto carbon black, 20 mL of NaBH 4 (6 mg/mL) was added into the suspension during vigorous stirring. The copper loading was 40 wt% for carbon black. In the case of Cu 2 O loading, the same procedure was performed, except that the solution pH for the suspension was adjusted to five by adding the sodium hydroxide solution. Finally, the reaction was completed for the stirring for 30 min after the addition of the reducing agent. The Cu and Cu 2 O-supported carbon black was collected by centrifugation (15,000 rpm) for five minutes, and the catalysts were washed several times with water and ethanol, and then were dried in oven at 60 • C overnight.

Electrode Fabrication
The drop-casting method was used for the electrode fabrication. First, 10 mg of Cu or Cu 2 O-supported carbon black was added with 4.96 mL of isopropanol and 40 µL of Nafion 117 solution (5 wt%), and was dispersed by ultrasonic irradiation. This suspension (25 µL) was dropped on a carbon plate (10 × 10 × 0.5 mm 3 ), and was dried for two hours. The loading of the catalyst was 0.05 mg/cm 2 .

Electrochemical Reduction of CO 2
The conditions for the electrochemical CO 2 reduction are given in Table S1 in the Supplemental Material. The electrochemical CO 2 reduction was carried out in a laboratory-made H-type cell. The cathode and the anode compartments were separated by an ion exchange membrane (Aldrich Nafion 117-type, 0.18-mm thickness). A silver rod coated with an insulator, except for the tip, was used as a quasi-reference electrode (Q.R.E.). The Ag Q.R.E. has a potential of about +0.100 V versus Ag/AgCl-saturated KCl that was connected with the catholyte.
Methanol (FUJIFILM Wako Pure Chemical Corp., >99%) was used as received without further purification. Sodium chloride was used as the supporting electrolyte in the methanol. Mechanical treatment of the platinum anode required polishing each surface with successively finer grades of alumina powder (Baikalox emulsion, manufactured by Baikowski International Co.) down to 0.05 µm, followed by the removal of grease with alcohol. The Pt anode was electrochemically polished in phosphoric acid by oxidation. Before the electrochemical CO 2 reduction, the anode was washed with both water and ethanol.
A potentiostatic electroreduction procedure (potential constant mode) was applied. First, CO 2 gas was bubbled for two hours while stirring the methanol catholyte. Next, the CO 2 -saturated solution was electrolytically reduced in the cathode potential range of −1.7 to −2.0 V (versus Ag Q.R.E.). The Faraday efficiencies of the main reduction product formations were calculated from the total charge passed of 50 Coulomb.
The gaseous products, which were collected in a homemade gas collector, and the liquid products, in the catholyte, were analyzed by gas chromatography (GC) and high performance liquid chromatography (HPLC), respectively.

Physicochemical Characterization
The crystal structure of the as-synthesized catalysts was analyzed by XRD technique, and their patterns are given in Figure 1. When the Cu 2+ ion in the suspension at pH 2 was reduced by NaBH 4 , the characteristic peaks appeared at two theta angles-43.  Figure 2. The large Cu blocks with a diameter of 500 nm were observed in the limited area in the SEM images, and were located onto the coagulation of carbon black. However, the TEM images showed that very fine Cu particles with an average size of 10 nm could be obtained onto the carbon black surface, as illustrated in Figure 2b. Therefore, it was confirmed that nanosized Cu particles could be deposited onto the surface of carbon black.

Effect of Potential
First, the current-potential curves were measured in CO 2 and N 2 -saturated methanol. The scan sweep rate for the potential was 50 mV s −1 . The typical current-potential curve for the Cu-deposited carbon black electrode is shown in Figure S1 in the Supplemental Material. The onset (starting) potentials of the cathodic current-that is, those potential values at which a current density of 0.1 mA cm −2 is observed in CO 2 -saturated methanol-was about −0.7 V. Since further CO 2 reduction may proceed with increasing the negative potentials, any voltammetric peak could not be observed in the potential range down to −2.0 V. After the onset, potentials could be estimated from the current-potential curve experiment, the influence of potential on the electrochemical CO 2 reduction at Cu or Cu 2 O deposited carbon black cathodes was investigated. Figures 3-5 shows the effect of applied voltages toward the electrochemical CO 2 reduction with prepared electrodes. Figure 3 displays that an increase in current density with increasing the applied voltage showed the rate of reaction on the electrode surface. Methane, ethylene, carbon monoxide, and methyl formate were the principal products identified by the analytical system used in the present work. First, formic acid was produced on the cathode surface by the electrochemical reduction of CO 2 . Then, formic acid can be partially converted to methyl formate in methanol, and it should be considered that methyl formate may be under the dissociation equilibrium of methanol and formic acid. The electrochemical activity of Cu or Cu 2 O-deposited carbon black to reduce CO 2 was investigated under a series of potentials. As shown in Figure 4, hydrocarbons (methane and ethylene) were observed at all of the tested voltages. The Faradaic efficiencies of carbon monoxide and methyl formate roughly increased as the potential decreased. On the other hand, the Faradaic efficiency of hydrogen increased. Ethylene was almost constant at all of the potentials (approximately 13%). The current efficiency curve for CH 4 was convex, and the maximum Faraday efficiency for methane was obtained at −1.9 V (26.9%). Figure 5 shows a similar trend to those obtained with Cu 2 O-deposited carbon black. At −1.9 V, the Faradaic efficiency of methane was maximum, and the efficiency was 30.6%. The amount of ethylene produced was constant (about 5%). Table S2 in the Supplemental Material shows the results for the electrochemical CO 2 reduction using the Cu plate electrode and the bare carbon black electrode under the same conditions. Only carbon black loading electrodes could not produce any hydrocarbons. In the electrochemical reduction of CO 2 in methanol at the Cu plate electrode, the total Faraday efficiencies of hydrocarbons were 23.0%. It was found in the present study that the hydrocarbon generation efficiencies at the Cu 2 O-deposited carbon black electrode were better than those obtained with the Cu plate electrode. The results mean that the electrocatalytic performance for hydrocarbon formation per Cu amount in this work was much better relative to that obtained conventionally with the Cu plate electrode.

Effect of Loading Amount
The influence of copper and copper oxide content on the electrochemical reduction of CO 2 at the modified carbon black electrode is shown in Figures 6 and 7. Both the Cu and Cu 2 O contents scarcely affected the current densities. For the metallic copper-deposited carbon black electrode, the total formation efficiencies for hydrocarbons increased with the content of Cu up to 40 wt%, and the efficiencies was 40%. Then, the efficiencies for hydrocarbons decreased at 50 wt%. On the Cu 2 O-deposited carbon black electrode, the maximum Faradaic efficiencies for total hydrocarbons were 40% at 40 wt% Cu 2 O. Consequently, 40 wt% Cu and 40 wt% Cu 2 O were optimum loading amounts for the formation of hydrocarbons in the electrochemical reduction of CO 2 in methanol at the modified carbon black electrode.   Temperature   Figures 8 and 9 show the influence of the catalyst preparation temperature on the electrochemical reduction of CO 2 in the methanol at modified carbon black electrodes. The catalyst preparation temperature scarcely affected the current densities. In the case of the Cu-deposited carbon black electrode, a low temperature for catalyst preparation improved the formation of hydrocarbons in the electrochemical CO 2 reduction in the methanol. The results may be due to the finer Cu particles produced in the low temperature. In the case of Cu 2 O-deposited carbon black, the Faradaic efficiencies for the products by the electrochemical reduction of CO 2 had similar trends to the electrode fabricated under various catalyst preparation temperatures.

Reaction Mechanism
According to the results in Table S2, the electrochemical reduction of CO 2 with bare carbon black particles could not produce any hydrocarbons. In the systems, methyl formate and CO were formed by the reduction of carbon dioxide. Furthermore, it was confirmed that the hydrocarbons were produced under all of the conditions by loading copper on the carbon black.
The Faradaic efficiencies for methane and ethylene at Cu 2 O/carbon black electrode were better relative to the results obtained with the copper plate, indicating that the charge transfer by carbon particles is very effective. The role of Cu species is illustrated in Figure 10. Although the electrochemical reduction of CO 2 in water and organic solvent at copper-based electrodes has been actively investigated, the catalytic mechanism could not be clarified, and is under the investigation. The adsorbed CO species are the first intermediate formed during the electrochemical reduction of CO 2 , and these CO species are very important for the formation of hydrocarbons. Montoya et al. [23] reported it that in a recent electrokinetic study, *CH x O intermediate is favored in the C-C coupling reaction rather than the *CO intermediate. However, the evidence for the direct observation of *CH x O could not be reported thus far.
The main difference for the electrochemical reduction of CO 2 in methanol electrolyte between the copper metal foil electrode and Cu 2 O (or Cu)-deposited carbon black electrode was the current efficiencies for methane and ethylene formation. The difference may be owing to contributions of the hydrogen molecule on the carbon support. Roughly, the excess hydrogen may introduce the electrochemical reduction of CO 2 toward methane and ethylene generation. Furthermore, carbon black may enhance the electron transfer mobility with metallic copper and Cu 2 O. As a consequence, the combination of metallic copper and Cu 2 O was effective for the improvement of hydrocarbon formation in the electrochemical reduction of CO 2 in the methanol-based electrolyte.

Conclusions
Cu and Cu 2 O-supported carbon black (Vulcan XC-72) nanoparticles were prepared by using the chemical reduction method for the deposition of Cu ion into carbon black, and the drop-casting method was applied for the fabrication of a modified carbon black electrode. The Cu or Cu 2 O loading amount and the electrode preparation temperature could be optimized for the electrochemical reduction of CO 2 in the methanol. The maximum Faradaic efficiency of hydrocarbons was observed at −1.9 V with 40.3% (26.9% of methane and 13.4% of ethylene) on the Cu 2 O-deposited carbon black catalyst. On the other hand, hydrogen formation could be depressed to 34.7%. Since the copper content was very trace (20 µg Cu/cathode), the effective efficiency of Cu will become very high. For instance, although the production rates for methane and ethylene were about 0.65 mmol min −1 kg −1 −Cu and 0.19 mmol min −1 kg −1 −Cu at the Cu plate electrode, for the Cu 2 O/carbon black electrode, they were approximately 24,000 mmol min −1 kg −1 −Cu and 8000 mmol min −1 kg −1 −Cu, respectively. The developed electrode will provide a clear framework for the future advancement of electrochemical conversion of CO 2 . Furthermore, carbon-supported copper oxide nanoparticles may be applied into the hybrid materials for carbon dioxide adsorption [24].
Supplementary Materials: The following are available online at http://www.mdpi.com/2305-7084/3/1/15/s1, Figure S1: Current-potential curves in CO 2 -saturated and N2-purged methanol, Figure S2: Reaction mechanism of the electrochemical reduction of CO 2 at the Cu and Cu 2 O-deposited carbon black electrodes in methanol, Table S1: Apparatus and experimental conditions, Table S2: Effect of electrode on the Faradaic efficiencies of products by the electrochemical reduction of CO 2 , Table S3: Comparison of the present electrode with other electrodes that were reported previously.
Author Contributions: N.U. and S.K. conceived and designed the experiments. N.U. performed the experiments and wrote the paper. I.T., M.F. and H.K. analyzed the results and advised the project.
Funding: The present research was partly supported by Grant-in-Aid for Scientific Research (C) 15K00602 from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.